وَإِنْ قَيَّدَ الْخَامِسَةَ بِسَجْدَةٍ: بَطَلَ فَرْضُهُ، وَتَحَوَّلَتْ صَلَاتُهُ نَفْلًا، وَكَانَ عَلَيْهِ أَنْ يَضُمَّ إِلَيْهَا رَكْعَةً سَادِسَةً.
Translation: If the fifth rakah is fixed by a prostration, the fard prayer is lost, the prayer turns into a voluntary prayer, and a sixth rakah must be joined to it.
Here the decisive threshold is fixing the fifth rakah with prostration. Without the required final sitting, prostration in the extra fifth prevents completion of the four-rakah fard prayer.
Under this Hanafi ruling, the prayer becomes nafl and a sixth rakah is added; the fard prayer must still be performed.